1) Field of the Invention
The present invention relates to a semiconductor laser device and a semiconductor laser module using the device.
2) Description of the Related Art
Conventionally, a semiconductor laser device as a light wave oscillator which utilizes photon induced emission due to an optical transition of electrons in a semiconductor crystal is widely known. Such a semiconductor laser device generally includes a resonator and oscillates a laser through light amplification by the resonator.
As the most basic structure of the resonator, the structure which adopts a Fabry-Perot resonator composed of two parallel reflection surfaces is known. Since the Fabry-Perot resonator can be easily formed by cleaving the semiconductor crystal composing the semiconductor laser device or polishing the semiconductor crystal into a mirror finished surface, it is widely used when using a semiconductor laser device as an ordinary light source.
However, in an optical communication system to be utilized for data communication represented by Internet or the like, the semiconductor laser device having the Fabry-Perot resonator is insufficient. In the optical communication, it is necessary to increase the intensity of a peak of a laser beam emitted from a semiconductor laser device to be used as a signal light source or an excitation light source in an emission wavelength and to reduce a half-width of the peak. Particularly in a dense-wavelength division multiplexing (DWDM) communication method which has been developed in recent years, it is necessary to use a laser beam having a plurality of wavelengths in a narrow wavelength range, and a half-width of peaks of the respective laser beams should be very narrow. It is extremely difficult to emit such a laser beam having a narrow half-value of a peak from the semiconductor laser device using the Fabry-Perot resonator.
For this reason, there suggests a semiconductor laser module which is provided additionally with a fiber grating in an optical fiber. FIG. 13 shows a structure of such a semiconductor laser module. This semiconductor laser module is composed of a structure such that a semiconductor laser device 101 is optically coupled with an optical fiber 104 via a first lens 102 and a second lens 103. In order to carry out laser oscillation, the resonator is composed of a reflection side end surface 106 and a fiber grating 105 arranged in the optical fiber 104. Since the fiber grating 105 has a sharp wavelength selection characteristic, the structure shown in FIG. 13 makes a peak of an oscillation laser beam sharp theoretically, and thus the peak intensity increases.
Alternatively, there also suggest a semiconductor laser device in which a diffraction grating is arranged in a vicinity of an active layer so as to generate a DFB (distributed feed back) laser and a DBR (distributed bragg reflector) laser which carry out single mode oscillation, and a semiconductor laser device in which a diffraction grating is arranged so as to generate a plurality of longitudinal oscillation modes mainly having a constant wavelength. Such semiconductor laser devices do not use a Fabry-Perrot resonator but uses a resonator having an additional excellent wavelength selection characteristic so as to obtain a sharp peak. This point is common to the semiconductor laser module using a fiber grating.
However, it is known that the semiconductor laser devices which include the resonator having the sharp wavelength selection characteristic have a problem. The above-explained semiconductor laser devices and laser module (hereinafter, referred to as “semiconductor laser device and the like”) normally have an emission side end surface 107 in a laser emitting direction as shown in FIG. 13. The emission side end surface 107 and the reflection side end surface 106 form the Fabry-Perot resonator. Therefore, an actual semiconductor laser device and the like have a structure such that an additionally provided resonator having a sharp wavelength selection characteristic and the conventional Fabry-Perot resonator coexist. In order to suppress amplification by the Fabry-Perot resonator, a single-layer film made of alumina (Al2O3) or the like is laminated on the emission side end surface 107 so that a reflectivity of a laser beam is reduced on the emission side end surface 107. However, actually a reflectivity of about 1 to 5% remains and an influence of the Fabry-Perot resonator cannot be eliminated completely.
On the contrary, a multi-layer film (hereinafter, referred to as “equivalent single-layer film”) which is optically equivalent to the single-layer film is laminated on the emission side end surface so that a reflectivity of a laser beam is made to be zero on the emission side end surface. This technique is disclosed in Japanese Patent Application Laid-Open No. 5-243689 (1993) (hereinafter, referred to as “conventional art”). The reflectivity on the emission side end surface becomes zero when the following equation is established,nf=(nsno)1/2  (1)wherein nf is refractive index of the single-layer film, ns is an effective refractive index of a semiconductor crystal composing the semiconductor laser device, and n0 is a refractive index of an external air contacting with the emission side end surface. When n0 is equal with one since it is a refractive index of air, the refractive index of the single-layer film is a square root of the effective refractive index of the semiconductor crystal. For example, when the semiconductor crystal composing the semiconductor laser device is made of InP, since ns equals to 3.25 when an emission wavelength λ is 1480 nm, nf nearly equals to 1.8. Since a material which satisfies the equation nf nearly equals to 1.8 has a problem in a strength and the like when the single-layer film is used as an actual transmission film, a plurality of films having predetermined refractive index and thickness are laminated so that an equivalent film of which entire refractive index becomes 1.8 is formed.
However, the conventional art has a problem. A wavelength of a laser beam oscillated from the semiconductor laser device occasionally deviates from a design value. When it is so, even when the equivalent single-layer film is formed according to a calculated value disclosed in the conventional art, the reflectivity zero cannot be realized, and thus there is a problem that the reflectivity becomes large. Since the refractive index is a function with respect to a wavelength of a transmitting light, the wavelength of the laser beam deviates so that the refractive index of plural materials forming the film also changes. Therefore, the entire refractive index also changes naturally, and the refractive index of the equivalent single-layer film obtains a value different from the refractive index realizing entire reflection. When the reflectivity due to this problem can be suppressed to an allowably low value, there arises no problem.
However, the equivalent single-layer film of the conventional art has a problem that a light of specified wavelength is not reflected but a light of a wavelength other than the specified wavelength is reflected greatly and thus the film functions as a cut filter as signified by the inventors. In general, when scattering of a laser beam emitted from the semiconductor laser device and scattering of film forming are taken into consideration, it is necessary to allow an error of about ±100 nm with respect to the design value. Therefore, the above conventional art has a problem in view of this point, and since the reflectivity of the emission side end surface with respect to a light having deviated wavelength becomes high, the Fabry-Perot resonator is formed.
In addition, the equivalent single-layer film of the conventional art has a problem of controlling thicknesses of the plural films to be laminated. In the conventional art, the equivalent single-layer film is composed of a multi-layer film made of the respective films having 90.23 nm or 8.25 nm thickness, but when the film is grown by electron beam vacuum evaporation, sputtering or the like, it is necessary to take the possibility of lamination error of about ±5% into consideration. Therefore, the actually laminated equivalent single-layer film does not occasionally have the reflectivity of zero due to the error of the lamination thickness. For this reason, in order to suppress the forming of the Fabry-Perot resonator effectively, it is necessary to previously design a structure such that even when the error of the lamination occurs, the reflectivity can be suppressed to a very low value.
In view of these problems, the film to be laminated on the emission side end surface of the semiconductor laser device should have a reflectivity of zero or an extremely low reflectivity with respect to a wavelength which is deviated by a certain amount from the specified wavelength, and even when the lamination error occurs in the film growth, a fluctuation of the reflectivity should be small. However, a study in view of this has not been particularly made until now.